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Tether_propulsion.
High voltage characteristics of the electrodynamic tether and the generation of power and propulsion final report (SuDoc NAS 1.26:1789 by P. Roger Williamson
Tether propulsion
Artist's conception of satellite with a tether
Tether propulsion uses long, strong strings (known as Tethers) to change the orbits of spacecraft. They have the potential to make space travel significantly cheaper.
Most current tether designs use crystalline plastics such as Spectra. A possible future material would be carbon nanotubes, which have theoretical strengths up to 100GPa (though recent (2004) experiments suggest that 60GPa may be more accurate).
There are four potential ways to use tethers for propulsion.
Table of contents showTocToggle("show","hide")
1 Tidal stabilization
2 Electrodynamic Tethers
3 Rotovators
4 Beanstalks, or Space Elevators
5 Problems
6 External Links
Tidal stabilization
An attitude control tether has a small mass on one end, and a satellite on the other. Tidal forces stretch the tether between the two masses, stabilizing the satellite so that its long dimension is always oriented towards the planet it is orbiting. Several of the earliest satellites were stabilized this way, or used mass distribution to get tidal stabilization. This is a simple form of stabilization that uses no electronics, rockets or fuel. A small bottle of fluid must be mounted in the spacecraft to damp vibrations.
Electrodynamic Tethers
An electrodynamic tether conducts current in order to act against a planetary magnetic field. It's a simplified, very low-budget magnetic sail.
It inducts Earth's magnetic field electric force to use as power and produce substantial work. When the conductive tether is trailed in a planetary or solar magnetosphere (magnetic field), the tether cuts the field, generates a current, and thereby slows the spacecraft into a lower orbit. The tether's end can be left bare, and this is sufficient to make contact with the ionosphere and allow a current to flow. The Phantom loop can be used as a circuit in electrodynamic tethers. A cathode tube may be place at the end of the tethers, also. The cathode tube will interact with the planet's magnetic field in the vacuum of space. A double-ended cathode tube tether will allow alternating currents.
A similar concept was used in Nikola Tesla's Patent on the wireless transmission of energy.
Electrodynamic tethers will build up vibrations from variations in magnetic and electric fields of the earth. The vibrations grow large enough so the tether will fail in less than a month from mechanical stress.
One plan to control these is to vary the tether current to oppose the vibrations, in simulations this keeps the tether together. The sensors to sense tether vibrations can either be an inertial navigation system on one end of the tether, or satellite navigation systems mounted on the tether, transmitting their positions to a receiver on the end. By channelling direct current through a tether, the spacecraft can be moved into a higher orbit.
An important patented application of an unpowered electrodynamic tether is to deorbit decommissioned satellites without the weight and complexity of a retrorocket. The tether deployment can be as simple as a spring, tidal forces then stretch the tether and orient the satellite as described above.
Jupiter rotates so rapidly that a tether can produce power and raise orbit passively and simultaneously
[1].
Rotovators
Rotovators could theoretically open up inexpensive transportation throughout the solar system, as long as the net mass flow was toward the Sun. On airless planets (such as the moon), a rotovator in a polar orbit would provide cheap surface transport as well.
A rotovator is a rotating tether. A spacecraft in one orbit rendezvous with the end of the tether, latching onto it and being accelerated by its rotation. This is not free. The tether's angular momentum changes. They separate later, when the spacecraft's velocity has been changed by the rotovator.
In a planetary magnetic field, a rotovator can be an electrodynamic tether, and its angular momentum can be charged electrically from solar or nuclear power, by running current through a wire that goes the length of the tether. When the tether turns over, the direction of current must reverse to act against the magnetic field. Ultimately, such a tether pushes agains the angular momentum of the planet.
Rotovators can also be charged by momentum exchange. Momentum charging uses the rotavator to move mass from a place that's higher in the gravity well to a place that is lower in the gravity well. The energy from the falling weight speeds up the rotation of the rotavator. For example, it is possible to use a system of two or three rotovators to implement trade between the Moon and Earth. The rotovators are charged by lunar mass (dirt, if imports are not available) dumped on Earth, and use the momentum so gained to boost Earth goods to the Moon.
Simple rotavators in circular orbits often can't be used because real materials are too weak. In particular, an earth-to-orbit rotavator in a circular orbit cannot be built from practical materials. There are tricks that can be combined to make rotavators practical with weaker materials.
One trick for using weaker materials is to put the rotovator in an elliptical orbit. It would pick up a load at apsis (closest approach), then vary the tether length or attachment point to throw the load (from the top of the tether) at a later time into a higher orbit. This splits the speed-exchange into two parts, each contributing half of the final velocity. It reduces the necessary size, strength and weight of the tether dramatically. It might be called a "revovator" because it exchanges momentum in both revolution and rotation. Recharging such a rotavator is more complex, too.
Another trick to lower stresses is that rather than picking up a cargo from the ground, at zero velocity, a rotavator can pick up a moving vehicle and sling it into orbit. For example, a rotavator could pick up a Mach-12 aircraft from the upper atmosphere of the Earth, and move it into orbit without using rockets. It could likewise catch such aircraft, and lower them into atmospheric flight. This would save tons of fuel per flight, and permits both a simpler vehicle and more cargo.
An important practical modification of a rotovator would be to add several latch points, to get different momentum transfers. Another important modification would be to add a linear motor to the rotovator, to accelerate spacecraft. This would permit travel times to the outer planets that were measured in months, rather than years. This is a very valuable option, given that such performance otherwise requires extremely exotic spacecraft propulsion systems.
Beanstalks, or Space Elevators
A beanstalk (more formally a space elevator) is a rotovator powered by the spin of a planet. For example, on Earth, a beanstalk would go from the equator to geosynchronous orbit.
A beanstalk does not need to be charged as a rotavator does, because it gets the required energy directly from its planet's angular momentum. The disadvantage is that it is very much longer, and for many planets a beanstalk cannot be constructed from known materials. An Earth beanstalk would be at the limit of current known material strength (2004).
Beanstalks also have much larger amounts of potential energy than a rotavator, and if heavy parts should fail they might cause multiple impact events as heavy parts hit the earth at orbital speeds. Most anticipated cable designs would burn up before hitting the ground.
For a more extensive article on beanstalks, see space elevator.
Problems
Simple tethers are quickly cut by micrometeoroids. The lifetime of a simple, one-strand tether in space is on the order of five hours for a length of ten km. Several systems have been proposed to correct this. The U.S. Naval Research Lab has successfully flown a long term tether that used very fluffy yarn. This is reported to remain uncut several years after deployment. Another proposal is to use a tape or cloth. Dr. Robert Hoyt patented an engineered circular net, such that a cut strand's strains would be redistributed automatically around the severed strand. This is called a Hoytether. Hoytethers have theoretical lifetimes of tens of years.
Beanstalks and rotovators are currently limited by the strengths of available materials. Although ultra-high strength plastic fibers (Kevlar and Spectra) permit rotovators to pluck masses from the surface of the Moon and Mars, a rotovator from these materials cannot lift from the surface of the Earth.
Tethers have many modes of vibration, and these can build to cause stresses so high that the tether breaks. Oscillations can be sensed by radio beacons on the tether, or inertial and tension sensors on the end-points. Mechanical tether-handling equipment is often surprisingly heavy, with complex controls to damp vibrations. Over a few tens of days, electrodynamic tethers in Earth orbit can quickly build vibrations in many modes, as their orbit interacts with irregularities in magnetic and gravitational fields. Electrodynamic tethers can be stabilized by reducing their current when it would feed the oscillations, and increasing it when it opposes oscillations.
Several conductive tethers have failed from unexpected current surges. It may be that the Earth's magnetic field is not as homogeneous as some engineers have believed. Unexpected electrostatic discharges have cut tethers, damaged electronics, and welded tether handling machinery.
External Links
See spacecraft propulsion, magnetic sail.
The above article is adapted from from Wikipedia All Wikipedia article text is available under the terms of the GNU Free Documentation License
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